How to Grow Cold Hardy Rosemary in Gardens
Rosemary (Salvia Rosmarinus) is an evergreen perennial herb native to Mediterranean lands. However, agronomists have developed cold hardy rosemary varieties suitable to grow in colder areas.
Rosemary including cold hardy rosemary belongs to Lamiaceae (mint) family. Many other herbs also constitute this family. In fact, the name “rosemary” is derived from ” ros marinus”, a Latin word.
This article mainly focuses on growing cold hardy rosemary, its varieties, and uses.
Generally, rosemary plants including cold hardy rosemary, have wooden stems with needle-like leaves. Being evergreen plants, they bear green leaves almost throughout the year. The flowers bloom in different colors including blue, purple, white, and pink. These plants have fibrous rooting systems.
The other name of the plant is Anthos, which means “flower” in the Greek language. Botanists referred these plants as Rosmarinus officinalis until 2017. After that, they have reclassified this plant as Salvia Rosmarinus.
Types of Cold Hardy Rosemary Plants
We have explained four well known and most widely used cold hardy rosemary varieties below:
Alcalde cold hardy rosemary variety can survive in USDA Zones 6 – 9. However, it can also survive USDA Zones 5 upper ranges with sufficient protection. Charles Martin, an agronomist, introduced Alcalde cultivation.
Normally, you can expect this variety to withstand sub-zero winters. It sprouts wide, olive-green leaves and pale blue flowers. This variety grows upright 30-36 inches in height and spreads 24-26 inches wide.
The other name for Madalene Hill variety is hill hardy rosemary. Normally, this variety grows upright 36-40 inches in height. It sprouts out light blue springtime blooms.
Madalene Hill rosemary is officially rated for USDA zone 6. it can easily survive sub-zero degrees Fahrenheit. However, make sure to provide adequate winter protection, if you plan to leave the plant outdoors throughout the year.
Experts consider ARP rosemary to be the most cold-tolerant variety. It is hardy to USDA Zone 5 and can casually withstand sub-zero temperatures.
Normally, this variety grows about four feet tall and spreads about 3-4 feet wide. It blooms out bright blue flowers. The slim leaves usually have a greyish green shade.
Athens Blue Spire Rosemary
The University of Georgia discovered Athens Blue Spire Rosemary in 1998. This cold-hardy rosemary variety is yet to be available widespread in the commercial market.
Normally, this variety grows upright 2-3 feet in height and spreads about two feet wide. It sprouts bluish lavender color flowers. The pale leaves usually have a greyish green shade.
Athens Blue Spire Rosemary can survive in USDA zone 5.
Propagation of Cold Hardy Rosemary Plants
Generally, you have three options to propagate cold hardy rosemary. You can propagate cold hardy rosemary from seeds, cuttings or layering. You also have the option of purchasing live plants and transplanting upon your desire.
Fertilize the soil regularly to encourage the plants healthy growth.
Normally, the seed germination process is a bit difficult. So, most gardeners hesitate to use this method. However, you can try this method, to grow cold hardy rosemary.
First, scarify the seeds. This means, rub the seeds with sandpaper or nick the seed coat with a sharp knife.
Then soak the seeds in warm water for at least a day. Change the warm water at regular intervals to maintain the seeds tepidly. This helps to initiate the germination process earlier compared to normal ‘prolonged periods.
Drain the water and collect the seeds. Scatter them individually over the soil and cover them slightly with the same.
Make sure to maintain the soil moisture by slightly watering the seeds. Do not overwater so as to cause waterlogging.
You can expect germination approximately in 2-3 months.
From Stem Cuttings
This is the most widely used method to propagate cold hardy rosemary plants. It is advisable to start propagation during the spring season.
Using good quality shears, cut out 2-3 inches of the new soft shoot. Generally, newly grown shoots are more active than the established ones.
Clean out the bottom 2/3 portion of the cutting by removing leaves, stems, and other debris. Make sure at least 5-6 leaves are left in the upper portion. Then dip the bottom end in rooting gel.
Fill the potting container with well-draining soil and good quality potting mixture. After that, plant the cuttings upright in the container.
Cover the arrangement with a plastic wrap to retain the moisture. keep this arrangement in places that have indirect lighting.
Normally, you can notice fresh new growth, within a few weeks. After that, remove the covering and transplant the cutting in your gardens.
Generally, this method is identical to the cutting methods. However, the difference is, the “cuttings” remain attached to the mother plant temporarily.
Normally, rosemary spreads with elongated stems. Select a stem that can be bent easily to reach the ground without being damaged.
Bend the stem gently down to the earth’s surface. Pin it down to the earth. Make sure to leave at least 2-3 inches of stem on the other side of the pin. Remove the leaves, barks, and other items of the stem, under the pinning portion and also on either side of the pin (at least 1 / 2 inch)
Bury the pinned portion underground or cover it thoroughly with the soil. water the arrangement regularly. You can expect to see the new growth on the tip within a few weeks. By this time, the pinned portion of the stem would have sprouted new roots underground. After that, detach the stem as a whole from the parent by cutting it off.
You can either grow the new plant in the same location or transplant it to a new location.
Caring Cold Hardy Rosemary Plants
You can plant cold hardy rosemary in containers and overwinter it indoors, comfortably. This is the easiest way to maintain these plants in ice-cold climates.
If you see the plants grew unwieldy in pots, trim them 2/3s before shifting them indoors.
In the case of established plants on the ground, if fear their survival during wild winters, just dig them up and replant them in large containers. Then shift them indoors after trimming.
Do not forget to wash the outdoor plants with hose water before shifting indoors. This helps to remove associated insects and pests.
Plan these procedures in august or early September, in order to avoid outdoor winters. You will also have enough time for proceeding with plant acclimation if you prefer to.
If you Intent permanent outdoor planting, then choose a sunny and sheltered location (preferably near a building), while planting itself. For winter caring, prune the grown plants to about 2-3 inches, after the initial frost. After that, completely cover the plant and surrounding area with thick mulch (4-6 inch layers)
Indoor Caring of Cold Hardy Rosemary Plants
Generally, most plants including cold hardy rosemary grow slower in winter. Hence, the necessity of water is also less. This simply means, water the indoor plants if the top layer of soil is dry.
However, do not allow the container soil to dry out completely. This may lead to root kill, ruining the plant as a whole. Make sure the container drainage is adequate, to help the watering process.
Pests and Diseases
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Mostly, Pests infest the weaken plants including cold hardy rosemary. In spite of all the efforts, most indoor cold hardy rosemary plants grow a bit weakened.
You can help them grow comparatively healthier by following a strict plan. Water them nominally and grow them in airy places with adequate sunlight. In this way, you can prevent pests from bothering the plants naturally.
However, If the plant is infected by pests, treat it with good quality organic pesticides. You can also try neem oil for treating pests. It has a good reputation in the pest treatment industry.
Photo by NorbertNagel (Wikimedia Commons) (CC BY-SA 3.0)
Generally, powdery mildew infects all varieties of the cold hardy rosemary. Facilitating adequate air passage can help to prevent this disease. This is easy in the case of our door plants.
Normally, indoors don’t have an airy environment compared to outdoors. So, you can blow air from electric fans to these plants, a few hours every day. Also, keep them out of high humidity rooms like the kitchen or bathrooms.
In the case of powdery mildew infections, treat the plants with a good quality fungicide. Spray the liquid on both sides of the leaves.
You can try spraying cinnamon and rubbing alcohol mixture on affected leaves.
Uses of Cold Hardy Rosemary Plants
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Generally, most gardeners use cold hardy rosemary plants as ornamental plants. Landscape experts use these plants for xeriscape landscaping. Rosemary’s beautiful fragrant flowers provide striking looks to ground space.
Normally, these plants retain their attractiveness for several years. You can also prune them to form fanciful or geometric shapes. The cold hardy rosemary varieties are known for their durable and dense texture.
Photo by Atudu (Wikimedia Commons) (CC BY-SA 4.0)
Rosemary plants cater to the food industry in many ways. You can use its leaves for flavoring foods and stuffing chicken, lamb, turkey, Etc., They are one of the main ingredients in traditional Mediterranean cuisine.
Some star hotels roast rosemary leaves with vegetables or meat (Especially barbecued foods) for its mustard-like fragrance.
The Rosemary plants also cater to the fragrance and consumer products industries. Rosemary oil is the primary constituent in some types of body perfumes or room freshener sprays.
Consumer products industries use them to produce some types of shampoos and cleaning products.
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